Sn-doped In2O3 thin films [In2-xSnxO3: ITO] exhibit a remarkable combination of optical and electrical transport properties. These include a low electrical resistivity, which is typically in the order of 10−4 Ωcm. This property is related to the presence of shallow donor or impurity states located close to the host (In2O3) conduction band, which are produced by chemical doping of Sn+4 for In+3 or by the presence of oxygen vacancy impurity states in In2O3-x. The films also exhibit high optical transparency (>80%) in the visible range of the spectrum (P. P. Edwards, et al.; Dalton Trans., 2004, 2995-3002).
Transparent conductive coatings or layers which comprise ITO have many applications, including in liquid crystal displays, flat panel displays (FPDs), plasma displays, touch panels, printed electronics applications, electronic ink applications, organic light-emitting diodes, electroluminescent devices, optoelectronic devices, photovoltaic devices, solar cells, photodiodes, and as antistatic coatings or EMI shieldings. ITO is also used for various optical coatings, most notably infrared-reflecting coatings (hot mirrors) for architectural, automotive, and sodium vapor lamp glasses. Other uses include gas sensors, antireflection coatings, electrowetting on dielectrics, and Bragg reflectors for VCSEL lasers. Furthermore, ITO can be used in thin film strain gauges. ITO thin film strain gauges can operate at temperatures up to 1400° C. and can be used in harsh environments.
Due to the cost and scarcity of indium metal, the principle component of ITO, a stable supply of indium may be difficult to sustain for an expanding market for flat panel displays, solar cells, printed electronics and other applications. There is therefore an ongoing need to reduce the amount of indium or produce indium-free phases as alternative transparent conducting oxide materials for transparent conductor applications.
The United States Department of Energy (DoE) has outlined various important criteria to be met by transparent conducting oxide materials (TCOs) to be used in such applications. These key requirements for TCOs are outlined in the US DoE document “Basic Research Needs For Solar Energy Utilization”, Report on the Basic Energy Sciences Workshop on Solar Energy Utilization, 2005, page 194. That document indicates that TCOs play an important role in all thin-film solar cells, and that the key properties for high-quality TCOs are high optical transmission (high band gap for window materials), low electrical resistivity and high carrier mobility, low surface roughness (for most devices), good thermal and chemical stability, good crystallinity (for most devices), adhesion and hardness, and low processing cost. Commonly used n-type TCOs include indium tin oxide (ITO) and SnO2 (both available commercially coated on glass) and cadmium stannate (Cd2SnO4). Developing p-type TCOs is also an important goal, because it would open up more possibilities for thin-film device structures, particularly multijunction devices. Materials being investigated include CuAlO2, CuInO2, CuSrO2, and (N, Ga)-doped ZnO.
International application no. PCT/GB2009/000534 (WO 2009/106828) describes a process for producing transparent conducting films of doped zinc oxides by pulsed laser deposition (PLD). The resulting transparent films were found to have temperature-stable electrical and optical properties comparable to those of ITO, and are attractive for transparent conductor applications as they can be produced from inexpensive, abundant precursors, and are non-toxic. Advantageously, therefore, the films go some way to meeting the important DoE criteria for TCOs.
A. K. Das et al., J. Phys. D: Appl. Phys. 42 (2009) 165405 (7 pp) also relates to the production of zinc oxide-based films by PLD.
Although PLD is a very useful tool for the growth of oxides (and other chemically complex systems) by reactive deposition, and allows key research to be performed in exploratory chemical doping programmes, PLD has limited applicability in industry and has certain drawbacks. For instance, in PLD, a compacted solid state target must first be produced. Typically, this target material is synthesised by heating a solid mixture of zinc oxide and one or more other materials which contain the relevant dopant elements. After synthesis, the target material is compacted to form the target and then placed in the chamber of a PLD apparatus. Subsequently, a pulsed laser beam is focussed on the target material to generate a plasma plume, and the plasma is deposited on a substrate to form the transparent conducting film. The PLD process therefore involves several steps, and requires the separate synthesis and preparation of a precursor target material in advance of film deposition.
Furthermore, the nature of the PLD apparatus and process restricts the size of the substrate on which the film is deposited and, in turn, the coverage area of film that can be deposited on a substrate. Substrate size is limited, for instance, by the size of the chamber of the PLD apparatus, the width of the chamber entrance though which the substrate is introduced, and the size of the substrate holder inside the chamber. Accordingly, only relatively small substrates can be coated by PLD. Furthermore, the area of film deposition is limited by the width of the plasma plume that is produced in the PLD apparatus, and the degree to which the substrate is moveable (translatable) relative to the plume within the chamber. Only relatively small-area films can therefore be produced by PLD. For instance, a typical area of homogeneous deposition of thin film produced by a laboratory PLD system is around 0.5 to 1.0 cm2.
Furthermore, the PLD process can lead to films with a non-uniform composition, due to the fact that the PLD ablation plume consists of two components; a high-intensity, leading part, which is usually stoichiometric in target composition, and a lower intensity non-stoichiometric material.
Additionally, both the PLD apparatus and the PLD process are expensive, requiring a vacuum system and an excimer laser.
Finally, the PLD process is usually limited to the deposition of films onto flat surfaces and materials, which restricts the types of substrates that can be coated using PLD.
There is therefore an ongoing need to provide improved, low-cost and simplified processes, which can achieve wide area coverage and overcome the above-mentioned difficulties, and which can produce transparent conducting films that are viable alternatives to ITO, namely films which have low electrical resistivity and high optical transparency in the visible range of the spectrum, are made from inexpensive, non-toxic materials, and address the abovementioned criteria outlined by the US DoE.